This invention relates to a circuit for use with telephone exchange systems comprising known electronic subscriber loop interface circuits (SLIC) or Borsht hybrid circuits, and more particularly to a circuit for suppressing a tone from being transmitted over a carrier channel while allowing the tone to be transmitted to the calling subscriber loop wherein said tone may be utilized in signalling telephone call charges.
As is generally understood, SLICs and Borsht hybrid circuits provide signal conversion between a balanced two wire bidirectional subscriber loop transmission path and a pair of unidirectional transmission paths and are typically employed in telephone exchange systems to provide interface between a telephone carrier channel and a subscriber loop, including a telephone handset, at the central office equipment. U.S. Pat. No. 4,004,109 describes such hybrid circuits and their applications.
In some telephone exchange systems it may be desirous to apply short signal bursts or a tax tone impulse signal to the hybrid circuits coupled to the subscriber loop of a telephone calling party to operate a tax counter incorporated into the telephone in order to determine the telephone call charge. Typically this tax tone would be at a frequency different from the normal signal frequency band of the carrier channel. The problem associated with this scheme arises from the nature of contemporary solid state SLIC or Borsht hybrid circuits now being used.
In application, a signal transmitted over the carrier channel is applied at the unidirectional receive wire pair terminals of the SLIC or Borsht hybrid circuit to cause an related signal to appear at the balanced bidirectional subscriber loop terminal to be received by the subscriber. Further, a signal originating from the subscriber is applied to the hybrid from the bidirectional subscriber loop to cause an related signal to appear at the unidirectional transmission wire pair path of the hybrid to be transmitted over the carrier channel. The hybrid circuit, however, cannot distinguish the origin of a signal appearing at the two wire pair of the bidirectional subscriber loop. Hence, a signal received on a unidirectional wire pair and causing an equivalent output signal to appear at the two wire bidirectional subscriber loop also causes an equivalent signal to appear on the unidirectional transmit two wire pair. In normal operation, the received signals are suppressed from being retransmitted over the carrier channel by adding an appropriately phased portion of the signal on the receive wire pair directly to that on the transmit wire pair as is known. The conditions for exact cancellation involve the values of the two wire output impedance of the hybrid circuit and the subscriber loop impedance. Variations of either of these impedances from the design center value will result in some transfer of the signal from the unidirectional receive wire pair to the unidirectional transmit wire pair.
In a scheme now being considered the tax tone impulses may be applied to the unidirectional receive wire pair of the SLIC or Borsht hybrid circuit associated with the calling party. Although the cancelling method utilized in the hybrid circuits described above will tend to reduce the signal transferred to the unidirectional transmit wire pair (and hence to the called party) the actual reduction will be inadequate because the impedances at the tax tone frequency cannot be controlled to the necessary accuracy. Hence, there is a need for a circuit which will allow tax impulse signals to be applied to the subscriber loop of the calling party while suppressing transmission thereof to the called party which is independent of the subscriber line impedance of the calling party.